Generally, hydraulic power steering apparatuses using hydraulic pressure of a hydraulic pump have been used as power steering apparatuses for vehicles. Since the 1990's, electric power steering apparatuses using an electric motor have been eventually popularized.
In a conventional hydraulic steering apparatus, a hydraulic pump, which is a power source providing power for the hydraulic steering apparatus, is operated by the engine of a vehicle, and always consumes energy regardless of the rotation of a steering wheel. In an electric power steering apparatus, on the other hand, if a steering wheel rotates and generates torque, an electric motor which is operated by electric energy provides assistant steering power. Therefore, when the electric power steering apparatus is used, it is possible to improve energy efficiency in comparison with the hydraulic power steering apparatus.
FIG. 1 is a schematic view showing a configuration of a conventional electric power steering apparatus for a vehicle.
As shown in FIG. 1, the conventional electric power steering apparatus includes a steering system 100 extending from a steering wheel 101 to both wheels 108, and an assistant power mechanism 120 for providing assistant steering power.
The steering system 100 includes a steering shaft 102 having an end connected to the steering wheel 101 to rotate along with the steering wheel 101, and the other end connected to a pinion shaft 104 by a pair of universal joints 103. Further, the pinion shaft 104 is connected to a rack bar 109 through a rack-pinion mechanism 105, while both ends of the rack bar 109 are connected to both wheels of the vehicle by tie rods 106 and knuckle arms 107, respectively.
The rack-pinion mechanism 105 has a pinion gear 111 mounted on the pinion shaft 104, and a rack gear 112 formed on a peripheral surface of one end of the rack bar 109 and engaged with the pinion gear 111. When a driver operates the steering wheel 101, torque is generated in the steering system 100 and causes the rack-pinion mechanism 105 and the tie rods 106 to steer the wheels 108.
The assistant power mechanism 120 includes a torque sensor 121 for sensing torque applied to the steering wheel 101 by the driver so as to output an electric signal in proportion to the sensed torque, an electronic control unit 123 for generating a control signal depending on the electric signal transmitted from the torque sensor 121, and an electric motor 130 for generating assistant steering power based on the control signal transmitted from the electronic control unit 123.
Accordingly, in the electric power steering apparatus, the torque generated by the rotation of the steering wheel 101 is transmitted through the rack-pinion mechanism 105 to the rack bar 109. The assistant steering power which the electric motor 130 generates depending on the generated torque, is transmitted to the rack bar 109. In other words, the rack bar 109 is moved along an axis thereof by the torque generated in the steering system 100 and the assistant power generated from the electric motor 130.
In such an electric power steering apparatus, the steering sensor gives the electronic control unit information on the rotation angle or angular velocity of the steering wheel with respect to an initial position of the steering wheel in a driving vehicle. The electronic control unit carries out a roll control or an orbiting control.
FIG. 2 is a block diagram illustrating a method for measuring a steering angle of the conventional steering apparatus.
As shown in FIG. 2, in the method for measuring the steering angle of the conventional steering apparatus, a steering angle sensor 200 obtains steering information using an optical sensor and a slit disc, and transmits the obtained information to an electronic control unit 123. The electronic control unit 123 calculates a steering angle and an angular velocity of the steering wheel.
Further, the steering angle sensor 200 according to the conventional art includes the optical sensor 301 and the slit disc 303, as shown in FIGS. 3 and 4. A column switch including the optical sensor 301 is fixed to a steering column, and the slit disc 303 is mounted on the steering shaft 102 to rotate along with the steering wheel when the steering wheel rotates.
The slit disc 303 is interposed between a light emitting unit 401 and a light receiving unit 403 of the optical sensor 301 so as to rotate along with the steering shaft 102 when the steering wheel is operated. At this time, an electric signal is determined according to whether the light from the light emitting unit 401 is transmitted to the light receiving unit 403.
FIG. 5 is a view showing a waveform of output signals of the steering angle sensor according to the conventional art.
As shown in FIG. 5, there exists a difference in voltage according to whether the light from the light emitting unit is transmitted to the light receiving unit, or not. The electric signal resulting from the change of voltage is transmitted to the electronic control unit, so that the electronic control unit can measure the steering angle.
However, the steering angle sensor according to the conventional art has a problem in that the steering angles of 0 degree, ±360 degrees, and ±720 degrees are regarded as the same angle. In other words, there is a disadvantage in that the steering sensor cannot distinguish clockwise rotation and counterclockwise rotation.
In order to solve such problems, a high-performance absolute steering sensor should be used for measuring an absolute steering angle. However, since the absolute steering sensor is an expensive element, it causes an increase in manufacturing costs of the steering apparatus.
In order to solve the above-mentioned problems of the conventional art, the present applicant has developed a steering apparatus, which is disclosed in Korean Patent Application No. 10-2005-72953. However, there is a problem in that it is difficult to install the steering apparatus, disclosed in the above-mentioned application, in a narrow space.